1. Field of the Invention
This invention relates to a magnetic disc drive device, and more particularly to the construction of a hub for clamping and rotating a magnetic disc.
2. Related Art
An example of a conventional magnetic disc drive device, in this case a hard disc drive device, is shown in FIG. 3. In FIG. 3, a cylindrical bearing holder 62 is mounted, for example, by press fitting in a boss formed in the center of a frame 60. A shaft 68 is rotatably mounted between lower and upper bearings 66, 66 inside the bearing holder 62. The shaft 68 is fixed in a hole in the center of the base of a cylindrical hub 70, for example, by press fitting whereby the hub 70 is supported in such a manner that the hub 70 can rotate integrally with the shaft 68. The purpose of the hub 70 is to hold a number of magnetic discs 82 which are integrally clamped to rotate together with the hub. The hub has at one end (the lower end in the drawing) a peripheral flange 72 for holding the magnetic discs 82 and has at the other end a number of threaded disc clamp holes 75. A suitable number of magnetic discs 82 are laminated with a suitable number of spacers in such a manner that the spacer is interposed between the magnetic discs 82. A clamper 84 is mounted on top of this stack, and the magnetic discs 82 are clamped to the hub 70 by fixing screws 86 which pass through the clamper 84 and are screwed into the threaded disc clamp holes 75.
A cylindrical drive magnet 76 is magnetized so that a different pole appears alternately in the circumferential direction and is fixed to the inner surface of the hub 70. A stator core 78 is fitted on the outer surface of the shaft holder 62. The stator core 78 has a plurality of salient poles. These salient poles 78 face the inner surface of the drive magnet 76, and a coil 80 is wound around each salient pole.
The drive magnet 76 and the parts which are integral with it make up the rotor of a disc drive motor, and the stator core 78 and the coils 80 make up the stator of this motor.
With this kind of disc drive device, vibration of the hub 70 during rotation is a problem. In particular, as advances are made in high-densification of recorded data, it is essential that the magnitude of the vibrations of the hub 70 in the radial and axial directions be reduced. In order to achieve this requirement, not only is it essential that each individual part be manufactured with a high degree of precision, but also post-assembly machining, in which the surface of the hub 70 is machined after the disc drive device has been assembled, is increasingly becoming necessary.
In post-assembly machining, while the hub 70 is rotated about its own axis, first the upper surface of the flange 72, on which the discs are mounted, is machined, and then the upper end surface of the hub 70 and then the outer surface 73 are machined, as shown in FIG. 4. Because the clamping force exerted by the clamper 84 on the magnetic discs 82 depends upon the height of the outer surface 73, i.e. the height dimension "H" between the upper surface of the flange 72 and the upper end surface of the hub 70, it is necessary to keep variations of the height dimension in a predetermined error range (for example to below .+-.0.05 mm). This is another reason why post-assembly processing is necessary.
In a conventional magnetic disc drive device, the whole upper end surface of the hub 70 is flat, and this whole flat surface has to be post-assembly machined. Therefore, it is necessary to machine the areas around the threaded disc clamp holes 75, because the threaded disc clamp holes 75 are formed in the upper end surface of the hub 70. The waste generated by the post-assembly machining enter the threaded disc clamp holes 75 and enter the motor section containing the stator core 78 and the drive magnet 76, etc. The waste can also get scattered over the magnetic discs 82 on the outside of the hub 70.
Also, in the post-assembly machining process, an axial screw hole 69 is provided in the bottom end of the shaft 68, and a male screw portion 92 of a post-assembly machining boss 90 is screwed into the axial screw hole 69 to couple the post-assembly machining boss 90 with the shaft 68. Thus, the pins 98 of a rotating member 96 are fitted into holes 94 formed in the boss 90 so that the shaft 68 and the hub 70 are rotated together with the rotating member 96. However, there has been the problem of this the post-assembly machining operation entailing a lot of work.